Micro-channel chemical concentrator

ABSTRACT

Apparatus and method for increasing the concentration of a chemical substance in a fluid comprise a micro-fluidic elongated channel formed in a substrate, with the channel being in fluid-flow communication with an ambient region along its elongated dimension. In general, the fluid includes first and second chemical substances having different vapor pressures. The apparatus includes an evaporation controller for increasing the evaporation rate of the fluid from the channel into the ambient region, thereby increasing the concentration of the higher vapor pressure (HVP) substance in the portion of the fluid remaining in the channel and increasing the concentration of the lower vapor pressure (LVP) substance in the portion of the fluid evaporated into the ambient region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to micro-channel chemical apparatus and methodsfor increasing the molar concentration of a chemical substance in afluid.

2. Discussion of the Related Art

In some applications it is necessary that reactive agents, such ashydrogen peroxide, be used in a highly concentrated form. When highlyconcentrated, however, some reactive agents exhibit short shelf lives,which means that only limited quantities of the agent may beinventoried. In addition, some highly concentrated reactive agents maybe unstable and/or unsafe.

Accordingly, there is a need in the art for a method and apparatus formaintaining such reactive agents in relatively dilute form and thenconcentrating them as needed.

In addition, it would be desirable for the apparatus to be realized in aminiaturized form, so as to make it more readily portable and reduce theexpense of the concentrating process.

BRIEF SUMMARY OF THE INVENTION

In accordance with one aspect of our invention, apparatus for increasingthe concentration of a chemical substance in a fluid comprises amicro-fluidic elongated channel formed in a substrate, with the channelbeing in fluid-flow communication with an ambient region along itselongated dimension. In general, the fluid includes first and secondchemical substances having different vapor pressures. The apparatusincludes an evaporation controller for increasing the evaporation rateof the fluid from the channel into the ambient region, therebyincreasing the concentration of the higher vapor pressure (HVP)substance in the portion of the fluid remaining in the channel andincreasing the concentration of the lower vapor pressure (LVP) substancein the portion of the fluid evaporated into the ambient region.

In accordance with another aspect of our invention, a method of alteringthe relative concentrations of first and second chemical substanceshaving different vapor pressures in a fluid, comprises the steps of:

-   -   (a) introducing the fluid into an input port of an elongated        micro-fluidic channel, the channel being in fluid-flow        communication with an ambient region along its elongated        dimension,    -   (b) causing the fluid to flow along the channel and to exit from        an output port, and    -   (c) increasing the evaporation rate of the fluid from the        channel into the ambient region, thereby increasing the        concentration of the HVP substance in the portion of the fluid        remaining in the channel and increasing the concentration of the        LVP substance in the portion of the fluid evaporated into the        ambient region.

In a preferred embodiment of both aspects of our invention, agas-permeable membrane is disposed between the channel and the ambientregion. The membrane confines the liquid form of the fluid to thechannel but allows the evaporated portion to flow therethrough into theambient region.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

Our invention, together with its various features and advantages, can bereadily understood from the following more detailed description taken inconjunction with the accompanying drawing, in which:

FIG. 1 is a schematic, cross sectional view of a micro-fluidicconcentrator in accordance with an illustrative embodiment of ourinvention;

FIG. 2 is a schematic, top view of a serpentine or zigzag channel of thetype depicted in the concentrator of FIG. 1;

FIG. 3 is a schematic view of one technique for increasing theevaporation of substances from fluid in the channel of a concentrator inaccordance with another embodiment of our invention;

FIG. 4 is a schematic, exploded view of a concentrator in accordancewith yet another embodiment of our invention;

FIG. 5 is a graph showing how the partial vapor pressure of hydrogenperoxide (H₂O₂) in aqueous solutions varies with temperature at variousH₂O₂ concentrations; and

FIG. 6 is a graph showing how the total vapor pressure of hydrogenperoxide (H₂O₂) in aqueous solutions varies with temperature at variousH₂O₂ concentrations.

The data of FIGS. 5-6 is taken from the website www.h2o2.com, whichcites Van Laar, Z Physik. Chem., Vo. 72, p. 723 (1910). The latter isincorporated herein by reference.

DETAILED DESCRIPTION OF THE INVENTION

In the following description we deal in general with a fluid thatincludes a HVP chemical substance and a LVP chemical substance. Turningnow to FIG. 1, we show an illustrative embodiment of our invention, amicro-fluidic concentrator 10 for increasing the molar concentration ofthe substances by preferentially evaporating the HVP substance.

Concentrator 10 comprises a substrate 12, an elongated fluid-flowchannel 14 formed in the substrate 12, an input port 16 for allowingfluid 14.1 a to be introduced from fluid source 20 into one end ofchannel 14, and an output port 18 for allowing fluid 14.1 c to beextracted from another end of channel 14. The extracted fluid is passedto a utilization device 22, which, for example, may simply be acollection vessel.

Channel 14 is configured to be in fluid-flow communication with anambient region 24, which in one embodiment is contained within acollection chamber 26. By fluid-flow communication we mean that gas orvapor 14.1 e that evaporates from fluid 14.1 b in channel 14 can flowdirectly or indirectly into region 24. By directly we mean that gas orvapor 14.1 e evaporating from fluid 14.1 b flows into region 24 withouttraversing any other components of the concentrator. On the other hand,by indirectly we mean that gas or vapor 14.1 e evaporating from fluid14.1 b flows into region 24 but first traverses at least one othercomponent of the concentrator. FIG. 1 illustrates the latter case; thatis, a gas-permeable membrane 32 covers the top of channel 14 and servesseveral purposes: first, to confine the fluid to the channel, which inturn allows the fluid to be forced (e.g., pumped) through the channel,and second, to allow gas or vapor 14.1 e to pass therethrough intochamber 24. In a preferred embodiment, the fluid 14.1 is a liquid, andthe membrane 32 is permeable to gas but not to liquid. Illustratively,the membrane is made of a polymer (e.g., PDMS or a photoresist), aporous inorganic solid (e.g., porous silicon, which can be made by thewell-known process of dissolving a single crystal silicon wafer in anelectrochemical cell containing a hydrogen fluoride solution; in thisprocess porosity can be tuned by varying the current applied to thecell), or a nanostructure of the type described, for example, by J. Kimet al., IEEE Conf MEMS, Las Vegas, Nev., pp. 479-482 (January 2002), M.S. Hodes et al., copending U.S. patent application Ser. No. 10/674,448filed on Sep. 30, 2003, and A. Komblit et al., copending U.S. patentapplication Ser. No. 10/403,159 filed on Mar. 31, 2003, all of which areincorporated herein by reference.

The concentrator 10 includes an evaporation controller for increasingthe evaporation rate of the fluid 14.b from the channel 14 into theambient region 24, thereby increasing the concentration of the LVPsubstance in the portion of the fluid 14.1 b remaining in the channeland increasing the concentration of the HVP substance in the portion ofthe fluid evaporated into the ambient region. The evaporation rate maybe enhanced in several ways in order to decrease the amount of timerequired to concentrate the LVP substance: for example, by heating thefluid, by lowering the pressure above the fluid in the channel, byflowing a gas over the fluid in order to minimize the vapor pressure ofthe LVP component in the ambient, and by proper design of the channel.These techniques may be applied singly or in any combination with oneanother and may be applied to those embodiments employing agas-permeable membrane (as shown in FIG. 1) as well as to those thatomit the membrane. In each of these techniques more of the HVP substancethan the LVP substance evaporates into the ambient 24, which increasesthe concentration of the LVP substance in the portion of the fluidremaining in the channel and increases the concentration of the HVPsubstance in the portion of the fluid that evaporates from the channelinto the ambient region.

Thus, the evaporation controller illustratively comprises a heater 28thermally coupled to substrate 12, and hence to the fluid 14.1 b inchannel 14. The heater 28 is typically a thermoelectric module, and asuitable electronic controller (not shown) supplies electric current toelectric terminals 30 to maintain the fluid temperature at the desiredvalue. The heater may be operated in either a continuous or pulsed mode,as is well known in the art. In one embodiment of heating in a pulsedmode, the heater comprises a multiplicity of heating elements that aredisposed along the length of the channel, and the heaters are pulsed inthe sense that they are activated sequentially. Due to the densitygradients it creates, pulsed mode heating also serves to mix the fluid.

The evaporation rate can also be increased by lowering the pressure inambient region 24 or by blowing a gas over the liquid-vapor interface.For example, when ambient region 24 is contained within chamber 26, avacuum may be established by using pump 34 to lower the pressure withinthe chamber. Alternatively, as shown in FIG. 3, the evaporation rate ofthe fluid in the channel is increased by flowing a gas 40 (air or aninert gas from source 42) directly across the surface of the channel 14or across the surface of the membrane 32.

Finally, the evaporation rate may be increased by proper design of theshape of the channel 14. In particular, it is advantageous to increasethe ratio of the surface area of the channel to its volume. Since theevaporation rate is known to be proportional to the area of the fluidsurface from which evaporation is taking place (i.e., the surface areaof the fluid exposed to ambient region 24), increasing the exposedsurface area increases the evaporation rate. To this end, a preferredembodiment of our invention utilizes a serpentine or zigzag shape of thechannel 14.2, as shown in FIG. 2. More specifically, the dimensions ofchannel 14 are determined by the evaporation rate per unit area of theinjected liquid and the volume of required concentrated liquid percycle.

Evaporation can occur in either a continuous or discrete cycle. In acontinuous cycle, low molar liquid is injected and evaporatedcontinuously, and high molar liquid is released at the output. In adiscrete cycle, low molar liquid is injected into the channel,evaporated and then pushed out of the channel by a newer batch of liquidin a periodic cycle. Note that the liquid can also slowly be injectedduring the evaporation process to make up for the mass loss.

An evaporation cycle is taken to be the time required to increase theconcentration of the HVP substance in the liquid phase to the desiredvalue. In a flow system the required time corresponds to the residencetime of a parcel of fluid flowing through the channel, but in a batchmode it corresponds to the time necessary for a batch of fluid to reachthe desired HVP concentration when the fluid is flushed into theutilization device 22.

We assume that the decomposition of the fluid 14.1 is low at the exposedinterface with ambient region 24 (e.g., at the interface betweenmembrane 32 and fluid 14.1) and that the evaporation rate of the LVPsubstance is, as a practical matter, independent of its concentration.In general, we want to maximize the surface area for evaporation if thedegradation of the fluid at the interface is low. Given, the evaporationrate and the desired concentration of the LVP substance at the outputport 18, one skilled in the art can readily calculate the exposed areaof the channel 14 (e.g., the top surface area of the serpentine channel14.2 shown in FIG. 2) and the duration of the evaporation cycle.Designing channel 14 to have a serpentine geometry allows for uniformheating and cooling of the fluid 14.1 by providing a large contact areabetween the fluid and the substrate.

The length of the channel and its cross sectional area determine thevolume of fluid that can be concentrated at any one particular time. Thechannel length on the other hand is determined in part by the size ofthe substrate surface in which the channel is formed. In designs thatutilize relatively expensive substrates, such as silicon, overall costconsiderations come into play when deciding whether to increase the sizeof the substrate in order to realize longer channels.

In those embodiments that employ collection chamber 24, it may bedesirable to condense the gas or vapors 14.1 e of the evaporant, whichincludes an increased concentration of the HVP substance. In oneembodiment, condensation is achieved by means of a cooler 38 thermallycoupled to chamber 24. The condensate is collected and stored inreservoir 36. Illustratively, the cooler is a thermoelectric coolerdriven by an electronic controller (not shown) that supplies current toelectric terminals 39. Other forms of coolers well known in the art mayalso be utilized. Alternatively, the cooler may be positioned proximatethe exit conduit 37, so that condensation takes place primarily in theconduit 37 rather than in the chamber 24. Another design of thisembodiment is shown in FIG. 4, which will be discussed infra.

To illustrate the operation, let us assume that the fluid 14.1 is aliquid, which is in dilute form (14.1 a) when introduced into thechannel 14 via input port 16 but is in concentrated form (14.1 c) whenextracted from output port 18. By dilute and concentrated we mean thatthe molar concentration of the LVP substance is higher at the outputport than at the input port. The liquid in the channel 14 is then heatedto a temperature below its boiling point to stimulate evaporation of theliquid from the channel 14 into the ambient region 24, therebyincreasing the concentration of the LVP substance in the liquidremaining in the channel. Temperatures above the boiling point arepreferably avoided in order to prevent bubble formation, which mightclog the channel and to prevent thermal breakdown of the chemicalsubstances in the fluid. Next, the concentrated liquid is cooled (e.g.,to room temperature), and the liquid is flushed from the channel intoutilization device 22. Flushing may be achieved by forcing air oranother liquid into the input port 16 or by using a mechanical plunger.

The electronic controller mentioned earlier may be employed to controlnot only the current supplied to the heater, 28 and the cooler 38, butalso to control any sensors (e.g., those that sense fluid temperature),pumps or fluid sources coupled to the concentrator, etc.

Another embodiment of our micro-channel chemical concentrator 50 isshown in FIG. 4. Here, the concentrator 50 includes a heating layer 52having a serpentine channel 52.1 and a heater 58 (FIGS. 4A and 4C), acooling layer 54 having a serpentine channel 54.1 and a cooler 59, aswell as a collection cavity 54.2 in fluid-flow communication withchannel 54.1 (FIGS. 4B and 4C). A gas permeable membrane 56 is disposedbetween the heater layer 52 and the cooling layer 54. Preferably, theheater 58 and the cooler 59, as well as their associated channels 52.1and 54.1, respectively, are axially separated from one another so thatthere is no (or very little) overlap between them, thereby reducing thelikelihood that the heating and cooling steps will interfere with oneanother.

The heater 58 is positioned adjacent the channel 52.1. It may comprise aresistive heater built into layer 52, or it may comprise heat tapeattached to the surface of layer 52. Likewise, the cooling layer 54comprises a channel 54.1 for cooling the fluid, and/or it may include anexternal cooler 59 positioned adjacent the channel 54.1.

In this embodiment of our invention, the liquid substance, which is tobe concentrated, has a higher vapor pressure than the solvent liquid. Aportion of the liquid is evaporated from the heater layer 52, passesthrough a membrane 56, and enters the cooling layer 54, where theevaporated portion is condensed and collected in cavity 54.2.

ILLUSTRATIVE APPLICATION

High molar hydrogen peroxide (H₂O₂) is a perishable and reactivechemical, which has a limited shelf life. A low molar concentration ofH₂O₂ is desirable because in this form the chemical is more durable andtransportable, as well as safer than at higher molar concentrations. Formany applications, where a higher molar concentration of H₂O₂ is needed,it is desirable to convert a low molar concentration to a high molarconcentration at or near the time when the chemical is to be used.

The illustrative application that follows describes increasing theconcentration of H₂O₂ in an aqueous solution. Thus, in the terminologyof the previous description the fluid 14.1 is a liquid, the HVPsubstance is water, and the LVP substance is H₂O₂. Various materials,dimensions and operating conditions are provided by way of illustrationonly and, unless otherwise expressly stated, are not intended to limitthe scope of the invention.

More specifically, our calculations show that our micro-channelconcentrator can be used to distill hydrogen peroxide from, for example,a 3% molar concentration to a >10% molar concentration in time scales onthe order of a few minutes. We considered conversion of 1 microliter or1 mm³ of liquid. An illustrative channel is, for example, 500 μm wide,250 μm deep and 8 mm long. In this case, the surface area of the channelis 4 mm², but it can have an arbitrary footprint in practice. FIGS. 5-6show the partial and total vapor pressures, respectively, of H₂O₂ as afunction of temperature for different concentrations. In general, thevapor pressure of H₂O₂ is more than ten times smaller than that ofwater. By heating an aqueous solution of H₂O₂, water is preferentiallyevaporated into air (i.e., into ambient region 24), resulting in a moreconcentrated H₂O₂ solution at output port 18. Assuming we start with a10% H₂O₂ solution, only about 1% of the vapor 14.1 e is H₂O₂, whichmeans that there is little loss of H₂O₂ in the concentrating process forsolutions that initially have low concentrations of H₂O₂.

We know that the evaporation rate for H₂O₂ increases exponentially withincrease in temperature and linearly with increase in the concentrationin the liquid at any temperature. The loss of H₂O₂ is found to beapproximately 10% of the concentration of H₂O₂ in solution atconcentrations of interest and temperatures below the boiling point ofthe solution. So long as the LVP component of the fluid has a smallervapor pressure than the HVP component of the fluid, evaporation isfavorable insofar as concentrating the LVP component. In the H₂O₂—H₂Oexample this implies we can concentrate H₂O₂ far beyond 10 wt % insolution.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments that can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention. In particular, it may be useful tocoat the walls of the channel for any number of reasons; e.g., toprevent corrosion or degradation of the channel; to prevent theformation of bubbles in the fluid and hence to prevent clogging; or toimprove thermal conductivity. For example, in the designs where thechannel is formed in a silicon substrate and a solution of H₂O₂—H₂O isto be concentrated, we know that H₂O₂ attacks silicon and would degradethe channel, so we need to coat the silicon channel with a layer ofprotective material such as silicon dioxide.

1. Apparatus comprising: a substrate, a micro-fluidic elongated channelformed in the substrate, said channel being in fluid-flow communicationwith an ambient region along its elongated dimension, an input port forintroducing a fluid into said channel and an output port for extractingfluid from said channel, said fluid comprising a high vapor pressurefirst substance and a low vapor pressure second substance, and anevaporation controller, said controller being configured to increase theevaporation rate of said fluid from said channel into said ambientregion, thereby increasing the concentration of said second substance inthe portion of said fluid remaining in said channel and increasing theconcentration of said first substance in the portion of said fluidevaporated into said ambient region.
 2. The apparatus of claim 1,further including a collection chamber that includes said ambientregion.
 3. The apparatus of claim 2, further including means forcondensing the portion of said first substance that is collected in saidchamber.
 4. The apparatus of claim 1, further including a gas-permeablemembrane disposed between said channel and said ambient region, saidmembrane confining said fluid to said channel but allowing saidevaporated first substance to flow therethrough to said ambient region.5. The apparatus of claim 4, wherein said membrane comprises a polymeror a porous inorganic solid.
 6. The apparatus of claim 1, wherein saidevaporation controller comprises a heater coupled to said substrate forsupplying heat to said the fluid in said channel.
 7. The apparatus ofclaim 6, wherein said controller operates said heater in a pulsed mode.8. The apparatus of claim 1, wherein said evaporation controllercomprises means for reducing the pressure of said ambient region.
 9. Theapparatus of claim 1, wherein said evaporation controller is configuredto blow a gas across the interface between said fluid and said ambientregion.
 10. The apparatus of claim 1, wherein said channel has aserpentine shape.
 11. The apparatus of claim 1, further including acoating formed on the surfaces of said channel.
 12. Apparatus comprisinga first substrate including a serpentine first channel disposed towardone end thereof, an input port for introducing fluid into said firstchannel, said fluid including a high vapor pressure first substance anda lower vapor pressure second substance, a second substrate including aserpentine second channel disposed toward the opposite end thereof andfurther including a collection cavity in fluid-flow communication withsaid first and second channels, a gas-permeable membrane disposedbetween said first and second substrates, said membrane being influid-flow communication with said first channel, means for heating saidfluid in said first channel to cause a portion of said fluid toevaporate and pass through said membrane into said cavity, and means forcooling said fluid portion in said cavity to cause it to condense, saidcondensed fluid flowing into said second channel.
 13. The apparatus ofclaim 12, further including a coating formed on the surfaces of saidchannels.
 14. A method of altering the concentrations of a higher vaporpressure first substance and a lower vapor pressure second substance ina fluid, comprising the steps of: (a) introducing the fluid into aninput port of an elongated micro-fluidic channel, said channel being influid-flow communication with an ambient region along its elongateddimension, (b) causing the fluid to flow along the channel and to exitfrom an output port, and (c) increasing the evaporation rate of saidfluid from said channel into said ambient region, thereby increasing theconcentration of said second substance in the portion of said fluidremaining in said channel and increasing the concentration of said firstsubstance in the portion of said fluid evaporated into said ambientregion.
 15. The method of claim 14, further including the step ofcollecting and condensing the portion of said fluid that evaporates intosaid ambient region.
 16. The method of claim 14, wherein saidevaporation rate increasing step includes heating the fluid in saidchannel.
 17. The method of claim 16, wherein said heating step operatesin a pulsed mode.
 18. The method of claim 14, wherein said evaporationrate increasing step includes reducing the pressure of said ambientregion.
 19. The method of claim 14, wherein step (c) includes the stepof blowing gas across the interface between said fluid and said ambientregion.
 20. The method of claim 14, wherein a gas-permeable membrane isdisposed between said channel and said ambient region, said membraneconfining said fluid to said channel but allowing said evaporated firstsubstance to flow therethrough to said ambient region,
 21. The method ofclaim 14, further including the step of collecting the portion of saidfluid remaining in said channel.